Involvement of Microglial P2Y₁₂ Signaling in Tongue Cancer Pain

Abstract

To elucidate if microglial P2Y12 receptor (P2Y12R) mechanisms are involved in the trigeminal spinal subnucleus caudalis (Vc; also known as the medullary dorsal horn) in intraoral cancer pain, we developed a rat model of tongue cancer pain. Squamous cell carcinoma (SCC) cells were inoculated into the tongue of rats; sham control rats received the vehicle instead. Nociceptive behavior was measured as the head-withdrawal reflex threshold (HWRT) to mechanical or heat stimulation applied to the tongue under light anesthesia. On day 14 after the SCC inoculation, activated microglia and P2Y12R expression were examined immunohistochemically in the Vc. The HWRT was also studied in SCC-inoculated rats with successive intra-cisterna magna (i.c.m.) administration of specific P2Y12R antagonist (MRS2395) or intraperitoneal administration of minocycline, a microglial activation inhibitor. Tongue cancer was histologically verified in SCC-inoculated rats, within which the HWRT to mechanical stimulation of the tongue was significantly decreased, as compared with that of vehicle-inoculated rats, although the HWRT to heat stimulation was not. Microglia was strongly activated on day 14, and the administration of MRS2395 or minocycline reversed associated nocifensive behavior and microglial activation in SCC-inoculated rats for 14 d. The activity of Vc wide dynamic range nociceptive neurons was also recorded electrophysiologically in SCC-inoculated and sham rats. Background activity and noxious mechanically evoked responses of wide dynamic range neurons were significantly increased in SCC-inoculated rats versus sham rats, and background activity and mechanically evoked responses were significantly suppressed following i.c.m. administration of MRS2395 in SCC-inoculated rats as compared with sham. The present findings suggest that SCC inoculation that produces tongue cancer results in strong activation of microglia via P2Y12 signaling in the Vc, in association with increased excitability of Vc nociceptive neurons, reflecting central sensitization and resulting in tongue mechanical allodynia.

Pathophysiological mechanisms of persistent orofacial pain

“…Recent studies have demonstrated that neuronal hyperexcitability in the orofacial pain transmission pathway contributes to persistent orofacial pain mechanisms following orofacial pathogenesis [4][5][6]. This pathway extends from the orofacial region to the pain information-processing region in the cerebrum and is associated with the pathological plastic changes in satellite glial cells in the trigeminal ganglion (TG), secondary neurons, microglia, and astrocytes in the Vc and C1-C2.…”

Purinergic Signaling and Dental Orofacial Pain

“…Microglial P2Y12 receptor is also reported to be involved in the central sensitization of orofacial pain [93]. In a tongue cancer, pain model produced by squamous cell carcinoma (SCC) cell inoculation, microglia were strongly activated in TSNC, and administration of MRS2395 or minocycline reversed the associated nociceptive behavior and microglial activation in SCC-inoculated rats.…”

P2Y12 Receptors in Tumorigenesis and Metastasis

“…This resulted to be associated with increased excitability of nociceptive neurons and consequent allodynia after mechanical stimulation. The administration of the P2Y 12 antagonist MRS2395 strongly reduced the “nocifensive” behavior and microglial activation in these animals ( Tamagawa et al, 2016 ).…”